As demand for mobile phone, mobile email, internet access, multimedia services, military communications and other broadband applications increases, the need for high data-rate and high capacity wireless systems increases. However, radio spectrum availability remains substantially constant. Therefore, it is desirable to increase spectral efficiency (more bits per second per hertz of bandwidth) and link capacity without the use of additional bandwidth.
Multiple-input multiple-output (MIMO) wireless technology employs multiple antennas at a transmitter and/or receiver to produce significant capacity gains over single-input single-output (SISO) systems, using the same bandwidth and transmit power. One aim of MIMO antenna design is to reduce correlation between received signals by exploiting various forms of diversity. For example, spatial diversity (spacing antennas apart), pattern diversity (using antennas with different or orthogonal radiation patterns), and polarization diversity (using antennas with different polarizations).
Spatial diversity requires that antennas are physically spaced apart. In order to achieve significant multiplexing and/or diversity gain, large spacing between multiple antennas is often required. Thus, spatial diversity can only be exploited when sufficient real estate is available. Pattern diversity typically requires multiple beam antennas that are capable of forming simultaneous beams in numerous different directions. Multiple bean antennas are typically complex, large, expensive, often have a limited bandwidth and typically have side lobes that can reduce their effectiveness.
Polarization diversity uses antennas with orthogonal polarizations, for example, horizontal and vertical, ±slant 45°, or left-hand and right hand circular polarization. Polarization diversity can also be achieved with three orthogonal antennas (e.g., tri-pole, tri-axial, tri-polarized, or triple axis antennas). Typically the three orthogonal antennas are co-located. Although polarization diversity can immunize a system from polarization mismatches that would otherwise cause signal fade, current polarization diversity antennas are not compact and do not have a wide frequency bandwidth.
Many civilian and military applications use Global Positioning Systems (GPS) and Global Navigation Satellite System (GNSS) for positioning, navigation, and timing. GPS and GNSS systems are susceptible to intentional and unintentional interference (e.g., jamming, a technique used by adversaries to distort signals). A Controlled-Reception-Pattern Antenna Array (CRPA) can form nulls to minimize interference, and can also suppress multipath signals. Existing CRPAs have limited bandwidth, rendering them unsuitable for use over numerous frequencies or over wide bandwidths (e.g., GNSS or GPS). CRPAs can also form very narrow nulls and are limited in the number of interfering sources that can be nulled. In addition, CRPAs are usually large (e.g., 35 centimeters in diameter) making them unusable on equipment that lacks sufficient mounting space (e.g., small missiles). Therefore, it is desirable to provide a compact, extremely wideband antenna, that can simultaneously null entire sectors of the sky.